As the demand for diesel fuel and aviation fuel increases worldwide there is increasing interest in sources other than petroleum crude oil for producing the fuels. One such source is what has been termed renewable sources. These renewable sources include, but are not limited to, plant oils such as corn, rapeseed, canola, soybean and algal oils, animal fats such as inedible tallow, fish oils and various waste streams such as yellow and brown greases and sewage sludge. The common feature of these sources is that they are composed of glycerides and free fatty acids (FFA). Both of these compounds contain aliphatic carbon chains having from about 8 to about 24 carbon atoms. The aliphatic carbon chains in the glycerides or FFAs can be saturated or mono-, di- or poly-unsaturated. The glycerides may be tri-glycerides, di-glycerides, mono-glycerides, or any combination thereof.
There are reports in the art disclosing the production of hydrocarbons from renewable sources. For example, U.S. Pat. No. 4,300,009 discloses the use of crystalline aluminosilicate zeolites to convert plant oils such as corn oil to hydrocarbons such as gasoline and chemicals such as para-xylene. U.S. Pat. No. 4,992,605 discloses the production of hydrocarbon products in the diesel boiling range by hydroprocessing vegetable oils such as canola or sunflower oil. Finally, US 2004/0230085 A1 discloses a process for treating a hydrocarbon component of biological origin by hydrodeoxygenation followed by isomerization.
Processes for producing two fuels, such as a diesel fuel and an aviation fuel, from renewable feedstocks are also known. The aviation fuel is produced via operation of the isomerization/cracking reactor in a higher severity mode to induce greater isomerization and cracking on longer chain n-paraffins (typically nC15-nC18), as well as catalyst selection.
As most natural oils that are used as feeds for making diesel and aviation fuel from renewable sources have fatty acid chain lengths of 16 to 18 carbon atoms, they are ideal for making diesel fuels. Deoxygenation of these oils results in C15-C18 normal paraffins which have poor cold-flow properties for diesel fuel. The normal paraffins are isomerized in an isomerization and selective hydrocracking reactor, in which the normal paraffins are converted into moderately branched isomers. This improves the cold-flow properties of the effluent fuel dramatically relative to the normal paraffins that resulted from deoxygenation.
However, in the production of aviation fuels from natural oils, the isomerization step must be run at higher severity to allow some diesel-range n-paraffins to be cracked or more heavily isomerized such that the resulting products boil in the jet range. This higher severity operation in the second stage results in product with lower boiling points and superior cold-flow properties than the feeds, but has the trade-off of producing increased amounts of naphtha and light ends at the expense of the aviation fuel component yield.
Therefore, there is a need for a process of producing aviation fuel at high yield from renewable feedstocks with lower quantities of naphtha and lights ends.